A morphological and genetic characterization of metamorphosis in the ascidian Boltenia villosa

Regulation and evolution of muscle development in tunicates

For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.

IAP genes partake weighty roles in the astogeny and whole body regeneration in the colonial urochordate Botryllus schlosseri

Inhibitors of Apoptosis Protein (IAP) genes participate in processes like apoptosis, proliferation, innate immunity, inflammation, cell motility, differentiation and in malignancies. Here we reveal 25 IAP genes in the tunicate Botryllus schlosseri's genome and their functions in two developmental biology phenomena, a new mode of whole body regeneration (WBR) induced by budectomy, and blastogenesis, the four-staged cycles of botryllid ascidian astogeny. IAP genes that were specifically upregulated during these developmental phenomena were identified, and protein expression patterns of one of these genes, IAP28, were followed. Most of the IAP genes upregulation recorded at blastogenetic stages C/D was in concert with the upregulation at 100 μM H₂O₂ apoptotic-induced treatment and in parallel to expressions of AIF1, Bax, Mcl1, caspase 2 and two orthologues of caspase 7. Wnt agonist altered the takeover duration along with reduced IAP expressions, and displacement of IAP28⁺ phagocytes. WBR was initiated solely at blastogenetic stage D, where zooidal absorption was attenuated and regeneration centers were formed either from remains of partially absorbed zooids or from deformed ampullae. Subsequently, bud-bearing zooids developed, in concert with a massive IAP28-dependent phagocytic wave that eliminated the old zooids, then proceeded with the establishment of morphologically normal-looking colonies. IAP4, IAP14 and IAP28 were also involved in WBR, in conjunction with the expression of the pro-survival PI3K-Akt pathway. IAPs function deregulation by Smac mimetics resulted in severe morphological damages, attenuation in bud growth and differentiation, and in destabilization of colonial coordination. Longtime knockdown of IAP functions prior to the budectomy, resulted in colonial death.

Formation of adult organs through metamorphosis in ascidians

The representative characteristic of ascidians is their vertebrate-like, tadpole shape at the larval stage. Ascidians lose the tadpole shape through metamorphosis to become adults with a nonmotile, sessile body and a shape generally considered distinct from that of vertebrates. Solitary ascidians including Ciona species are extensively studied to understand the developmental mechanisms of ascidians, and to compare these mechanisms with their counterparts in vertebrates. In these ascidian species, the digestive and circulatory systems are not well developed in the larval trunk and the larvae do not take food. This is in contrast with the inner conditions of vertebrate tadpoles, which have functional organs comparable to those of adults. The adult organs and tissues of these ascidians become functional during metamorphosis that is completed quickly, suggesting that the ascidian larvae of solitary species are a transient stage of development. We here discuss how the cells and tissues in the ascidian larval body are converted into those of adults. The hearts of ascidians and vertebrates use closely related cellular and molecular mechanisms that suggest their shared origin. Hox genes of ascidians are essential for forming adult endodermal structures. To fully understand the development and evolution of chordates, a complete elucidation of the mechanisms underlying the adult tissue/organ formation of ascidians will be needed. WIREs Dev Biol 2018, 7:e304. doi: 10.1002/wdev.304 This article is categorized under: Comparative Development and Evolution > Body Plan Evolution Early Embryonic Development > Development to the Basic Body Plan.

Recurrent phagocytosis-induced apoptosis in the cyclical generation change of the compound ascidian Botryllus schlosseri

Colonies of the marine, filter-feeding ascidian Botryllus schlosseri undergo cyclical generation changes or takeovers. These events are characterised by the progressive resorption of adult zooids and their replacement by their buds that grow to adult size, open their siphons and start filtering. During the take-over, tissues of adult zooids undergo extensive apoptosis; circulating, spreading phagocytes enter the effete tissues, ingest dying cells acquiring a giant size and a round morphology. Then, phagocytes re-enter the circulation where they represent a considerable fraction (more than 20%) of circulating haemocytes. In this study, we evidence that most of these circulating phagocytes show morphological and biochemical signs of apoptosis. Accordingly, these phagocytes express transcripts of orthologues of the apoptosis-related genes Bax, AIF1 and PARP1. Electron microscopy shows that giant phagocytes contain apoptotic phagocytes inside their own phagocytic vacuole. The transcript of the orthologues of the anti-apoptotic gene IAP7 was detected only in spreading phagocytes, mostly abundant in phases far from the take-over. Therefore, the presented data suggest that, at take-over, phagocytes undergo phagocytosis-induced apoptosis (PIA). In mammals, PIA is assumed to be a process assuring the killing and the complete elimination of microbes, by promoting the disposal of terminally differentiated phagocytes and the resolution of infection. In B. schlosseri, PIA assumes a so far undescribed role, being required for the control of asexual development and colony homeostasis.

The cAMP-binding Popdc proteins have a redundant function in the heart

Popdc (Popeye-domain-containing) genes encode membrane-bound proteins and are abundantly present in cardiac myocytes and in skeletal muscle fibres. Functional analysis of Popdc1 (Bves) and Popdc2 in mice and of popdc2 in zebrafish revealed an overlapping role for proper electrical conduction in the heart and maintaining structural integrity of skeletal muscle. Popdc proteins mediate cAMP signalling and modulate the biological activity of interacting proteins. The two-pore channel TREK-1 interacts with all three Popdc proteins. In Xenopus oocytes, the presence of Popdc proteins causes an enhanced membrane transport leading to an increase in TREK-1 current, which is blocked when cAMP levels are increased. Another important Popdc-interacting protein is caveolin 3, and the loss of Popdc1 affects caveolar size. Thus a family of membrane-bound cAMP-binding proteins has been identified, which modulate the subcellular localization of effector proteins involved in organizing signalling complexes and assuring proper membrane physiology of cardiac myocytes.

Metamorphosis in solitary ascidians

Embryonic and postembryonic development in ascidians have been studied for over a century, but it is only in the last 10 years that the complex molecular network involved in coordinating postlarval development and metamorphosis has started to emerge. In most ascidians, the transition from the larval to the sessile juvenile/adult stage, or metamorphosis, requires a combination of environmental and endogenous signals and is characterized by coordinated global morphogenetic changes that are initiated by the adhesion of the larvae. Cloney was the first to describe cellular events of ascidians' metamorphosis in 1978 and only recently elements of the molecular regulation of this crucial developmental step have been revealed. This review aims to present a thorough view of this crucial developmental step by combining recent molecular data to the already established cellular events.

Differential expression of proteins and phosphoproteins during larval metamorphosis of the polychaete Capitella sp. I

BACKGROUND

The spontaneous metamorphosis of the polychaete Capitella sp. I larvae into juveniles requires minor morphological changes, including segment formation, body elongation, and loss of cilia. In this study, we investigated changes in the expression patterns of both proteins and phosphoproteins during the transition from larvae to juveniles in this species. We used two-dimensional gel electrophoresis (2-DE) followed by multiplex fluorescent staining and MALDI-TOF mass spectrometry analysis to identify the differentially expressed proteins as well as the protein and phosphoprotein profiles of both competent larvae and juveniles.

RESULTS

Twenty-three differentially expressed proteins were identified in the two developmental stages. Expression patterns of two of those proteins were examined at the protein level by Western blot analysis while seven were further studied at the mRNA level by real-time PCR. Results showed that proteins related to cell division, cell migration, energy storage and oxidative stress were plentifully expressed in the competent larvae; in contrast, proteins involved in oxidative metabolism and transcriptional regulation were abundantly expressed in the juveniles.

CONCLUSION

It is likely that these differentially expressed proteins are involved in regulating the larval metamorphosis process and can be used as protein markers for studying molecular mechanisms associated with larval metamorphosis in polychaetes.

Trunk lateral cells are neural crest-like cells in the ascidian Ciona intestinalis: insights into the ancestry and evolution of the neural crest

Neural crest-like cells (NCLC) that express the HNK-1 antigen and form body pigment cells were previously identified in diverse ascidian species. Here we investigate the embryonic origin, migratory activity, and neural crest related gene expression patterns of NCLC in the ascidian Ciona intestinalis. HNK-1 expression first appeared at about the time of larval hatching in dorsal cells of the posterior trunk. In swimming tadpoles, HNK-1 positive cells began to migrate, and after metamorphosis they were localized in the oral and atrial siphons, branchial gill slits, endostyle, and gut. Cleavage arrest experiments showed that NCLC are derived from the A7.6 cells, the precursors of trunk lateral cells (TLC), one of the three types of migratory mesenchymal cells in ascidian embryos. In cleavage arrested embryos, HNK-1 positive TLC were present on the lateral margins of the neural plate and later became localized adjacent to the posterior sensory vesicle, a staging zone for their migration after larval hatching. The Ciona orthologues of seven of sixteen genes that function in the vertebrate neural crest gene regulatory network are expressed in the A7.6/TLC lineage. The vertebrate counterparts of these genes function downstream of neural plate border specification in the regulatory network leading to neural crest development. The results suggest that NCLC and neural crest cells may be homologous cell types originating in the common ancestor of tunicates and vertebrates and support the possibility that a putative regulatory network governing NCLC development was co-opted to produce neural crest cells during vertebrate evolution.

Expression pattern of Popdc2 during mouse embryogenesis and in the adult

The Popdc2 gene is a member of the Popeye domain containing gene family encoding membrane proteins with prominent expression in striated and smooth muscle tissue. After introducing a LacZ reporter gene into the Popdc2 locus, expression was studied during embryonic development and postnatal life. At embryonic day (E) 7.5, expression was present in cardiac and extraembryonic mesoderm. At E10.5, expression was found in heart, somites, and mesothelial cells lining the coelom. At E12.5, expression was present in the coelomic mesothelium, pericardial and myocardial layer of the heart, skeletal muscle, bladder, gut, and umbilical vessels. Postnatal expression was found in cardiac and skeletal muscle and in the smooth muscle layer of colon, rectum, and bladder. In the stomach, Popdc2 was exclusively present in the pyloric epithelium. In conclusion, Popdc2 is expressed in various muscle and nonmuscle cell types during embryonic development and in postnatal life.

A complement response may activate metamorphosis in the ascidian Boltenia villosa

Ascidian metamorphosis transforms a free-swimming larval chordate ascidian into a sessile adult through a distinct series of metamorphic events. Initially, larvae must become competent to respond to settlement cues. Settlement is then marked by dramatic body plan remodeling and may be accompanied by attachment to the substrate. Subtractive hybridization has revealed that many innate immunity transcripts are upregulated during metamorphosis in the ascidian Boltenia villosa. Several of these genes have well-known roles in the mannose-binding lectin (MBL)-complement pathway of innate immunity, including MBL and MBL-activated serine protease (MASP). MBL recognizes and binds to bacterial pathogens, activates MASP, and triggers the complement cascade. In B. villosa, larvae upregulate BvMASP at the time of competency to initiate settlement. We show that several bacterial strains can induce settlement and that the timing of BvMASP expression in the papillae-associated tissue (PAT) cells is tightly correlated with larval competency. We further demonstrate that serine protease inhibitors used to block the complement response also block metamorphosis, allowing tail resorption, but preventing further morphological changes. Based on these experiments, we propose that the MBL-complement pathway may be important for competency, bacterial substrate detection and body plan remodeling during metamorphosis.

Germline transgenesis and insertional mutagenesis in the ascidianCiona intestinalis

Stable transgenesis is a splendid technique that is applicable to the creation of useful marker lines, enhancer/gene traps, and insertional mutagenesis. Recently, transposon-mediated transformation using a Tc1/mariner transposable element Minos has been reported in two ascidians: Ciona intestinalis and C. savignyi. The transposon derived from an insect, Drosophila hydei, has high activity for excision in Ciona embryos and transposition in their genome. As much as 37% of Minos-injected C. intestinalis transmitted transposon insertions to the subsequent generation. Minos-mediated germline transgenesis has also been achieved by means of electroporation method. Minos techniques have been applied to enhancer traps and insertional mutagenesis in Ciona. For those reasons, Minos offers the high potential for use as a powerful tool for future genetic studies. This review specifically addresses recent achievements of transformation techniques in Ciona, as exemplified using the Minos system.

Comparative analysis of mRNA and protein expression of Popdc1 (Bves) during early development in the chick embryo

The isolation of the Popeye gene family was based on its preferential expression in striated muscle tissue. Recently, a monoclonal antibody against chick Popdc1 (also known as Bves) became available and was used in this study to comparatively analyze the expression pattern of Popdc1 at both the protein and mRNA level during early chick embryogenesis. Using whole-mount immunohistochemistry, expression in the heart was first observed at Hamburger and Hamilton (HH) stage 10 in the presumptive left ventricular segment. Cardiac expression was confined to differentiated cardiac myocytes, and undifferentiated myocytes at the anterior and posterior pole showed little expression. After looping, the outer curvature myocardium showed prominent Popdc1 staining, whereas the inner curvature was unlabeled. Despite previous reports, Popdc1 protein was not detectable at any time point in the proepicardium, epicardium, or the smooth muscle layer of the coronary vessels. Whole-mount in situ hybridization using a full-length Popdc1 probe detected novel expression domains, which have not been described previously. Popdc1 mRNA was found in Hensen's node at HH stage 4, and by HH stage 5+, expression became asymmetric. In addition, Popdc1 mRNA was found in pharyngeal endoderm and in the notochordal plate. Subsequently, beginning at HH stage 9, Popdc1 mRNA expression was found in the cardiac mesoderm and expression was maintained in the heart in a pattern very similar to the one observed by antibody staining.

Bves, a member of the Popeye domain-containing gene family

Bves was discovered through subtractive screens designed to identify heart-enriched transcripts. Bves is a transmembrane protein that possesses a highly conserved structure among species of the animal kingdom. Various approaches have been used to elucidate the expression pattern of Bves mRNA and protein as well as its function in developing and mature organisms. Emerging evidence indicates that this protein is present in muscle and epithelia of developing embryos and the adult. In vitro functional studies predict a role in cell-cell interaction and/or adhesion. In vivo analysis of protein function is very limited at present, but recent work in Xenopus supports the importance of Bves in epithelial integrity. Presented in this review is a compilation of published findings concerning Bves gene and protein characteristics, expression patterns in embryos and cells, and functional significance as determined thus far. Presently, the literature supports a hypothesis that Bves is essential to the junctional architecture of muscle and epithelial cell types. Although there remain aspects of Bves structure, expression, and function that are not completely resolved, now is an appropriate time to summarize current knowledge about this protein, the remaining questions, and what its potential role in development might be. This review will serve as a departure point for others who become interested in the study of this highly conserved protein.

Tissue-Specific Profile of DNA Replication in the Swimming Larvae of Ciona intestinalis

The cell cycle is strictly regulated during development and its regulation is essential for organ formation and developmental timing. Here we observed the pattern of DNA replication in swimming larvae of an ascidian, Ciona intestinalis. Usually, Ciona swimming larvae obtain competence for metamorphosis at about 4-5 h after hatching, and these competent larvae initiate metamorphosis soon after they adhere to substrate with their papillae. In these larvae, three major tissues (epidermis, endoderm and mesenchyme) showed extensive DNA replication with distinct pattern and timing, suggesting tissue-specific cell cycle regulation. However, DNA replication did not continue in aged larvae which kept swimming for several days, suggesting that the cell cycle is arrested in these larvae at a certain time to prevent further growth of adult organ rudiments until the initiation of metamorphosis. Inhibition of the cell cycle by aphidicolin during the larval stage affects only the speed of metamorphosis, and not the formation of adult organ rudiments or the timing of the initiation of metamorphosis. However, after the completion of tail resorption, DNA replication is necessary for further metamorphic events. Our data showed that DNA synthesis in the larval trunk is not directly associated with the organization of adult organs, but it contributes to the speed of metamorphosis after settlement.